Wireless communication networks based on the Long Term Evolution, LTE, specifications promulgated by the Third Generation Partnership Project, 3GPP, use an “Evolved Packet System” or EPS that includes an “Evolved Packet Core” or EPC, and an “Evolved UMTS Terrestrial Radio Access Network” or E-UTRAN. The interested reader may refer to the technical specification 3GPP TS 23.002 for an architectural overview of the EPC, and may refer to 3GPP TS 23.401 for EPC architectural details with respect to E-UTRAN access. Further, 3GPP TS 36.300 provides substantive details for the E-UTRAN.
For purposes of this discussion, it is sufficient to note that the E-UTRAN includes base stations referred to as enhanced NodeBs, which are also known as eNBs or eNodeBs. The eNBs provide the E-UTRA user-plane and control-plane protocol terminations towards the user equipments, UEs, operating in the network. The eNBs interconnect with each other through an “X2” interface, and interconnect with the EPC through an “S1” interface. More specifically, the eNBs interconnect to Mobility Management Entities or MMEs in the EPC by means of an S1-MME interface and to Serving Gateways or S-GWs in the EPC by means of an S1-U interface. The S1 interface supports many-to-many relations between given eNBs and respective SGWs and MMEs within the EPC.
An MME in the EPC operates as a control node and in that role it processes signaling between a UE and the EPC. Primary functions of the MME relate to connection management and bearer management, which are handled via Non Access Stratum, NAS, protocols. In particular, the NAS protocols support mobility and session management procedures for UEs operating in an LTE network. The NAS protocols form the highest stratum of the control plane between a UE and the EPC, which may be referred to more simply as a core network or CN. According to TS 24.301, NAS signaling connections are peer-to-peer S1 mode connections between UEs and MMEs, but this connection includes a Radio Resource Control, RRC, connection with the UE via the LTE air interface in the E-UTRAN, denoted as the “LTE-Uu” interface, along with a corresponding S1AP connection via the S1 interface.
While TS 24.301 provides a detailed presentation of the NAS protocols, it is useful here to note that the main functions of the NAS are the support of mobility and session management procedures for UEs, e.g., to establish and maintain Internet Protocol, IP, connectivity between a UE and a Packet Data Network Gateway, P-GW, in the CN. According to this broad functional understanding, the NAS protocols divide into categories; namely EPS Mobility Management protocols, referred to as EMM protocols, and EPS Session Management protocols, referred to as ESM protocols.
IP connectivity between a UE and a PDN is based on a PDN connection, i.e., through an S-GW and P-GW of the EPC, and an EPS bearer supported by the E-UTRAN air interface. The ESM protocols support network-initiated EPS bearer procedures, including activation, deactivation and modification. Further, the ESM protocols support UE-initiated transaction procedures, including PDN connection establishment and disconnection requests, bearer resource allocation and modification requests, and bearer release requests.
The EMM protocols include EMM common procedures, EMM specific procedures, and EMM connection management procedures. The EMM connection management procedures provide various functionality, including: network-initiated paging to indicate a NAS service request to a UE, UE-initiated service requests to initiate a NAS signaling connection, and the transport of various NAS messages. The EMM common procedures are network-initiated and include a number of functions, such as these items: reallocation of Global Unique Temporary IDs, security mode control, and authentication.
In contrast, the EMM specific procedures are UE-initiated and include attach/detach procedures, for attaching and detaching from the EPC. The EMM specific procedures also provide tracking functionality, wherein the EPC maintains an awareness of the locations of idle-mode UEs within the network. The tracking functionality is based on the network being divided into Tracking Areas, where different subsets of cells or eNBs in the E-UTRAN and associated subsets of MMEs and S-GWs represent different Tracking Areas. When a UE detects that it has entered a new Tracking Area, it sends a Tracking Area Update, TAU, message to the network. TAU messages may also be sent on a periodic basis, according to a TAU timer.
The attach, detach and TAU procedures, along with certain other NAS procedures, are performed for the purpose of exchanging specific information between the UE and the EPC, and are not followed by any other NAS or user plane procedures. Such NAS procedures generate a considerable amount of signaling traffic and consume battery life at the UE. Certain steps have been taken to reduce the amount of this type of NAS signaling going between individual UEs and the EPC. For example, a UE may be provisioned with a Tracking Area Indicator, TAI, list. In turn, the UE performs a TAU procedure only when it enters a cell that is identified as being in a Tracking Area not in its TAI list. Another method aimed at reducing the amount of such NAS signaling combines tracking area updates and location area updates.
However, certain trends in network design, deployment and usage result in networks with increasing numbers of cells and increasing numbers of UEs or other wireless devices, which in turn means more NAS signaling will be required more frequently. Examples of these trends include the proliferation of wireless devices, both in the consumer and industrial markets, and particularly the emergence of machine-type devices that use wireless communication networks for conveying Machine Type Communications between the machine-type devices and their respective Machine-to-Machine, M2M, service-provider networks.
Another example trend is network “densification,” which is based on using denser arrangements of network base stations, to improve coverage and increase data rates. The increasing use of “heterogeneous” networks represents another example trend. Heterogeneous networks are economically attractive as compared to network densification, which improves coverage simply by adding more wide-area base stations. In contrast, heterogeneous networks improve coverage and/or provider higher-date rate services based on adding additional access points that typically are lower-power and lower-complexity as compared to the “normal” network base station or access points used to provide wide-area cellular radio connectivity.
In an example heterogeneous network, one or more macro or large area cells are overlaid with one or more smaller cells, often generically referred to as “pico” cells, to denote their relatively small coverage areas, as compared to the macro-cell coverage areas. The pico cells may be used as hotspots that provide higher data rate service and/or may be used to extend the macro-cell coverage areas or to fill in macro-cell coverage gaps. Other pico cells may operate with closed subscriber groups, such as Home eNBs. More generally, a heterogeneous network includes a mix of base station or access point types, where the access point types may be distinguished by any one or more of: different access point powers and/or corresponding cell sizes, and different Radio Access Technologies or RATs.
For example, it is known to have a 3GPP Radio Access Network, RAN, such as the LTE E-UTRAN, overlaid with a non-3GPP RAN, such as provided by one or more Wi-Fi networks. The 3GPP RAN may be regarded as first access network that is directly coupled to and coordinated with the core network, while the Wi-Fi network(s) may be regarded as a second access network that may or may not having any integration with the core network.